THE 


AMERICAN  JOURNALOFSCIENCE 

[FOURTH  SERIES.] 


Art.  YII. — On  an  Artificial  Lava-Flow'  and  its  Spher- 
ulitic  Crystallization  •  by  L.  V.  Pirsson.  (With  Plate  I.) 


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The  material  which  forms  the  basis  of  the  following  article, 
was  obtained  a  number  of  years  ago  by  the  late  Prof.  Chas.  E. 
Beecher,  of  Yale  University,  at  Kane,  an  industrial  town  with 
glass-works,  in  McKean  county,  Penn.,  and  presented  to  the 
writer.  It  appears  that  one  of  the  furnaces  in  the  glass-works 
accidentally  broke,  allowing  the  molten  glass  to  escape ;  it 
flowed  into  the  pit  below  and  cooled  there.  When  cold  the 
mass  of  glass  was  blasted  out  and  the  broken  material  piled  on 
one  side.  It  was  from  this  broken  material  that  the  specimens 
were  obtained.  The  furnace  is  stated  to  have  been  60  feet 
long  by  25  feet  wide,  and  the  molten  glass  in  it  at  the  time  to 
have  been  from  4  to  5  feet  deep.  This  would  have  made 
about  6000  cubic  feet  of  glass,  or  say  700,000  pounds,  an 
amount  which  formed  a  flow  of  no  mean  size,  when  viewed 
from  the  experimental  standpoint.  During  the  flow  and 
while  cooling  it  partially  crystallized,  assuming  features  which 
it  is  the  object  of  this  paper  to  present.  An  accident  of  this 
kind  to  a  glass  furnace  by  which  a  flow  of  molten  glass  was 
produced,  which  partly  crystallized,  has  been  described  by  the 
late  Professor  Fouque.*  In  this  case  portions  of  the  glass  were 
filled  with  milky  white  nodules  with  a  greenish  cast  of  color, 
which  were  in  places  the  size  of  a  nut.  These  nodules  were 
spherulites  which  Fouque  proved  to  consist  of  radially  fibrous 
prisms  of  wollastonite.  Thanks  to  the  kindness  of  Professor 
A.  Lacroix  of  the  Musee  d’histoire  naturelle  in  Paris  the  writer 
has  been  able  to  examine  some  of  the  glass  described  by 
Fouque  and  to  confirm  his  conclusions.  The  present  material, 
while  resembling  it  in  some  respects,  differs  in  several  impor¬ 
tant  particulars,  as  will  be  presently  shown. 


* 


Compt.  Rendus,  cix,  Jan.  1,  1889. 


Am.  Jour.  Sci. — Fourth  Series,  Yol.  XXX,  No.  176. — August,  1910. 
7 


98 


L.  V.  Pirsson — Artificial  Lava-Flow 


Morozewicz*  lias  also  described  crystallizations  resulting 
from  the  outflowing  of  glass  from  industrial  furnaces;  in  this 
case  large  single  crystals  of  wollastonite  were  formed.  He 
mentions  also  rounded  aggregates  of  fibrous  diopside,  but  does 
not  describe  them  definitely  as  of  spherulitic  structure. 

Spherulites . — The  most  important  kind  of  crystallization 
shown  by  the  Kane  glass-flow  is  in  the  formation  of  spherulites. 
The  glass  itself  is  an  ordinary  green  bottle  glass,  pale  greenish 
where  thin  and  a  clear,  sea-green  when  a  couple  of  inches  thick 
or  more.  This  is  more  or  less  filled  with  white  spherical 
bodies  which  vary  in  size  from  that  of  very  fine  shot  up  to 
those  which  are  nearly  the  size  of  an  egg.  These,  and  espe¬ 
cially  the  largest  ones,  may  be  readily  broken  out  of  the  glass, 
either  as  single  spheroids,  or  as  grouped  or  botryoidal  masses. 
When  extracted  they  have  a  smooth,  thin,  outer  shell  of  glass 
which  covers  them  like  a  skin.  These  solid  bodies  when 
broken  open  are  white,  with  a  pale  greenish  tinge,  and  are 
seen  to  possess  a  fibrous  radiated  structure.  The  fibers  are 
extremely  fine  and  thread-like,  giving  to  the  surface  the  luster 
of  floss  silk.  This  is  true  even  in  the  largest.  In  some  the 
cross  section  of  the  broken  spherule,  as  it  lies  in  the  glass, 
appears  radial  but  uniform,  while  in  others  changes  in  the 
conditions  of  crystallization,  as  the  fibers  grew  outwardly  from 
the  center,  have  produced  a  series  of  concentric  rings,  such  as 
are  sometimes  observed  in  the  smaller  spherulites  in  rhyolite. 
As  many  as  ten  of  these  rings,  of  almost  perfectly  circular  form 
and  of  similar  width,  have  been  observed  in  one  spherulite, 
and,  thus  mottling  the  silky  sheen  of  the  surface  of  the  section 
of  the  divided  spherulite,  they  have  a  moire,  or  concentric, 
watered-silk  appearance,  and  add  greatly  to  its  beauty.  The 
structure,  or  consistency,  of  the  spherulites  varies  greatly ; 
some  are  so  compact  that  they  are  composed  almost  wholly  of 
crystalline  material,  while  in  others  the  thread-like  fibers  are 
relatively  widely  separated  and  the  whole  spherulite  is  satura¬ 
ted  with  the  greenish  glass.  Thus,  while  the  first  form  very 
solid  white  objects,  the  latter  appear  almost  like  misty  or  cloudy 
forms  in  the  green  matrix  surrounding  them,  and  the  con- 
choidal  fracture  passes  through  the  glass  without  reference 
to  them  nor  can  they  of  course  he  broken  out  and  separated 
from  it. 

In  another  portion  of  the  flow  which  formed  a  sheet  about 
two  inches  thick  the  spherulites  have  a  somewhat  different 
character.  Here  they  appear  to  be  composed,  not  of  tenuous 
fibers,  but  of  distinct,  rather  thick,  blades,  0*5-l*0mm  broad  by 
4mm  long,  as  may  be  seen  by  reference  to  figs.  B  and  C  in 
Plate  I  which  represents  them  in  very  nearly  natural  size. 

*Min.  Petr.  Mitt.,  xviii,  p.  124,  1898. 


and  its  Spherulitic  Crystallization. 


99 


They  average  about  7-8mm  in  diameter.  In  addition  to  the 
regular  spherulites  various  modifications  of  them  exist  in  this 
place,  as  may  be  seen  by  reference  to  the  plate.  Some  are 
composed  of  only  a  few  blades,  or  even  only  two  or  three, 
radiating  from  a  center  in  a  star-like  group,  but  with  blades 
the  same  size  and  length  as  those  of  the  more  complete  spher¬ 
ulites.  The  beauty  of  these  white  radiate  and  stellate  crystal¬ 
lizations  suspended  in  the  clear  sea-green  glass  is  very  striking, 
and  the  fact  that  they  are  thus  suspended  and  did  not  fall  to 
the  bottom  shows  that  at  the  time  of  their  formation  the  molten 
glass  was  in  too  viscous  a  condition  to  permit  of  such  move¬ 
ment,  a  fact  whose  bearing  upon  the  question  of  their  origin 
will  be  treated  later.  The  bottom  of  this  sheet  of  glass  is 
composed  of  a  layer  of  spherulites  about  3-4mm  in  thickness. 
Looking  at  the  bottom  surface  itself  it  appears  flat  and  smooth 
but  so  thickly  covered  with  the  radiate  crystallizations  of 
these  spherulites,  seen  in  half  section,  that  they  nearly  every¬ 
where  coalesce,  or  are  contiguous.'  The  appearance  recalls 
surfaces  covered  with  radiate  zeolites  or  frosted  window-panes 
in  winter.  The  upper  surface  of  this  layer  presents  a  some¬ 
what  mossy  appearance  as  the  rounded,  bladed,  surfaces  of  the 
spherulites  project  into  the  glass.  It  is  seen  on  the  right  hand 
side  of  figs.  B  and  C  in  the  plate.  The  white  cloudy  area  near 
the  top  of  the  piece  in  tig.  C  is  due  to  the  internal  reflection 
of  light  in  the  glass  and  nearly  conceals  a  very  fine  spherulite. 
The  blades  composing  these  spherulites  are  small  at  the  center 
where  they  unite  and  grow  larger  gradually  as  they  extend 
into  the  glass.  They  are  four  sided  with  apparent  right  angles 
and  are  terminated  by  an  oblique  plane,  but  the  crystallization 
is  rough  and  imperfect.  Close  inspection  of  the  photograph 
will  show  these  details. 

Mineral  Composition. — The  study  of  the  mineral  compos¬ 
ing  these  spherulites  in  thin  sections  and  in  powdered  grains 
under  the  microscope  proves  that  it  is  artificial  diopside.  This 
is  seen  from  the  following  properties:  inclined  extinction, 
c  on  o  measured  to  a  maximum  of  39° ;  sections  having  par¬ 
allel  extinction  show  the  exit  of  an  optic  axis  on  the  edge  of 
the  field  and  that  the  plane  of  the  optic  axes  lies  in  the  clino- 
pinacoid  parallel  to  the  length  of  the  prism ;  by  repeatedly 
measuring  the  width  of  the  nearly  square  prisms  as  they  lie  in  the 
glass  their  thickness  is  also  obtained,  and  the  birefringent  color 
yielded  by  those  having  the  maximum  angle  of  extinction  indi¬ 
cates  that  the  maximum  birefringence  is  about  0’03.  The  cross 
section  of  the  prisms  shows  them  to  be  nearly  square,  but  they 
are  too  minute  to  show  the  cleavage  parallel  to  the  prismatic 
sides  ;  the  extinction  bisects  the  angle.  These  are  the  proper¬ 
ties  of  diopside  and  the  physical  determination  was  confirmed 


100 


L.  V.  Pirsson — Artificial  Lava-Flow 


by  qualitative  chemical  tests  on  material  extracted  from  the 
glass  which  proved,  in  addition  to  the  silica,  the  presence  of 
traces  of  iron  and  alumina  and  abundant  lime  and  magnesia. 
It  is  evident  that  a  dolomite  limestone  was  used  in  the  making 
of  the  glass. 

Microstructure  of  Crystals. — When  studied  under  the 
microscope  in  powdered  form  it  is  found  that  the  crystal 
blades,  illustrated  in  the  photographic  plate,  are  very  far  from 
being  the  solid  continuous  crystals  they  appear.  Close  inspec¬ 
tion  of  them  with  a  simple  lens,  as  they  lie  in  the  glass,  proves 
them  to  have  a  parallel  fibrous  structure.  The  microscope 


Figs.  1,  2.  Skeleton  crystals  of  diopside. 

shows  them  in  length  sections  to  consist  of  bundles  of 
extremely  slender  rods  or  fibers.  These  are  sometimes  closely 
packed,  sometimes  separated  by  much  more  than  their  own 
diameters.  They  are  surrounded  by,  or  are  cemented  together, 
by  the  glass  in  which  they  lie.  Sections  across  these  fibrous 
bundles,  or  blades,  prove  that  to  a  great  degree  they  are  not 
simple  solid  fibers,  or  rods,  but  are  more  or  less  hollow,  or 
skeleton-like  in  form,  and  that  groups  of  them  have  a  similar 
crystallographic  orientation.  One  of  these  is  shown  in  the 
adjoining  fig.  1.  The  directions  of  extinction  in  this  are 
indicated  by  the  broken  arrow  lines ;  these  indicate  the  plane 
of  symmetry  in  the  compound  or  skeleton  crystal  and  it  is 
interesting  that  the  directions  of  growth  are  along  both  pina- 
coidal  and  prismatic  faces.  Another  type  is  shown  in  fig.  2  ; 
the  growth  here  is  along  the  clinopinacoid  l):  010,  and  the 
prismatic  faces ;  that  along  the  othopinacoid  being  wanting. 
A  number  of  different  patterns  of  growth  were  observed  but 
they  are  sufficiently  illustrated  by  these  examples. 


and  its  Spheriditic  Crystallization. 


101 

Cham  Spherulites. — An  interesting  feature  is  the  presence 
of  chain  spherulites  along  flowage  lines,  as  illustrated  in  the 
photographic  plate  fig.  A.  The  white  lines  are  composed  of 
layers  of  innumerable  minute  spherulites,  while  the  darker 
layers  between  are  of  clear  green  glass  free  from  crystalline 
products.  Examination  with  a  lens  reveals  the  fact  that 
almost  without  exception  these  lines  are  composed  of  a  chain, 
or  layer,  of  single  spherulites.  Generally  they  are  so  closely 
crowded  that  they  coalesce  to  a  considerable  extent,  less  often 
they  touch  at  the  circumference,  and  in  some  places  they 
appear  quite  regularly  spaced  but  separated  by  areas  of  glass, 
generally  not  wider  than  their  own  diameter.  Viewed  with 
the  lens  from  above,  a  layer  appears  like  one  of  minute  white 
pills  sifted  into  the  glass,  innumerable  in  number  and  spread 
without  order,  here  thickly  clustered,  there  more  thinly  distrib¬ 
uted.  The  average  size  of  these  spherulites  is  0'5-O6mm  and 
they  do  not  vary  much  from  it.  Under  the  microscope  in  thin 
section  they  appear  like  the  spherulites  of  feldspar  seen  in  acid 
lavas,  so  much  so  that  it  is  almost  difficult  to  believe  one  is  not 
dealing  with  a  section  of  rhyolite.  Although  snow-white  in 
the  glass,  and  also  as  seen  by  the  microscope  with  reflected 
light  from  the  section,  they  appear  of  a  light  leather-brown 
color  by  transmitted  light.  W ith  a  low  magnification  the  cen¬ 
tral  part  of  the  spherulite  looks  uniform  and  homogeneous,  the 
fibrous  character  appears  more  evident  as  the  outer  boundary 
is  approached,  and  the  outermost  zone  is  composed  of  distinct, 
branching,  fibrous  rays.  With  high  magnification  the  central 
part  is  also  resolved  into  masses  of  excessively  fine,  tenuous, 
matted  fibers.  At  the  center,  where  these  fibers  are  cut  per¬ 
pendicularly,  they  appear  as  mere  points. 

When  the  structure  of  these  bodies  is  revealed  by  the  use  of 
crossed  nicols,  it  is  seen  that  they  are  not  formed  uniformly 
of  single  fibers  radiating  from  a  common  center,  but  rather 
of  groups,  or  bundles,  of  fibers  somewhat  radially  divergent, 
like  brooms,  or  brushes ;  closely  stellate  groupings  of  which 
form  the  spherulites.  Thus  they  repeat  on  a  minute  scale  the 
examples  shown  in  B  and  C  of  the  plate  and  described  in  the 
preceding  section.  Like  other  spherulites  they  show  a  black 
cross,  which  is  sometimes  in  one  bundle,  or  brush,  parallel  with 
the  planes- of  the  nicols  and  sometimes,  as  the  spherulite  is 
revolved  and  a  new  brush  comes  into  position,  is  somewhat 
inclined,  proving  that  in  each  brush  the  majority  of  the 
minute  fibers  tend  to  have  orientated  positions  and  thus  make 
a  compound  skeleton  crystal.  This  is  also  like  the  larger  ones 
described  and  figured  in  the  preceding  section. 

The  light  leather-brown  color  of  the  sections  of  these  spher¬ 
ulites  is  a  curious  feature.  It  is  strongest  in  the  central  areas 


102 


L.  V.  Pirsson — Artificial  Lava-Flow 


where  the  fibers  are  finest  and  most  closely  packed ;  at  the 
periphery,  where  they  are  coarser  and  more  individualized,  it  is 
wanting.  With  a  very  high  magnification  of  the  center  and 
parallel  light  from  below,  it  disappears.  With  low-powered 
objectives  it  is  present,  whether  the  lower  nicol  is  present  or 
not.  The  spherulites  of  feldspars  in  acid  volcanic  lavas  often 
show  the  same  phenomenon  of  brown  coloration,  and  so  do  the 
closely  packed  aggregates  of  minute  scales  of  kaolin  in  feldspar. 
Since  in  all  these  cases  the  component  mineral  fibers,  or  scales, 
are  white,  that  is  to  say  colorless,  this  effect  cannot  be  due  to 
a  pigment,  but  must  be  an  optical  phenomenon  due  to  the 
absorption  of  light.  An  explanation  of  this  may  be  somewhat 
as  follows  :  Angular  fibers  or  scales  of  a  colorless  mineral 
lying  in  all  positions  in  a  medium  of  lower  refractive  index 
may  act  as  prisms  to  rays  of  light  passing  through  them.  The 
rays  of  white  light  would  be  then  decomposed  into  their  pri¬ 
mary  colors,  and  the  violet  rays  more  inclined  than  those  of  the 
red.  On  passing  upward  and  striking  inclined  surfaces  of  new 
fibers,  or  scales,  the  light  in  the  violet  end  of  the  spectrum,  on 
account  of  its  greater  inclination  than  that  towards  the  red 
end,  would  tend  in  greater  measure  to  suffer  total  reflection 
and  be  absorbed,  while  the  less  inclined  rays  of  the  red  and 
yellow  would  pass  through  and,  illuminating  the  dark  areas  of 
total  reflection,  give  the  fibrous  mass  the  brown  tone.  This 
explanation  is  based  essentially  on  the  same  principle  as  that 
given  by  von  Federov*  for  the  pseudo-pleochroism  which 
he  observed  in  minerals  containing  thin  parallel  laminations. 
Only  in  this  case  there  is  no  parallelism,  the  angular  fibers  lie 
in  all  directions,  hence  there  is  no  apparent  pleochroism  and 
the  mass  appears  of  the  same  brown  color  in  all  positions  and 
with,  or  without,  polarized  light. 

It  may  be  that  in  this  lies  the  explanation  of  the  brown  and 
gray  colors  and  pleochroic  effects  observed  in  spherulites  in 
certain  volcanic  glasses  by  Cole,f  but  a  search  through  the  lit¬ 
erature  has  not  yielded  to  me  any  discussion  of  the  frequent 
brown  color  of  spherulites  (or  kaolin)  though  so  commonly 
observed.  Probably  it  is  usually  ascribed,  if  noticed,  to  some 
ferruginous  pigment,  to  which  in  some  cases  it  is  undoubt¬ 
edly  due. 

Some  other  features  of  interest  which  these  spherulites  show 
are  as  follows  :  In  some,  especially  larger  ones,  hollow  cavities 
appear ;  others  have  no  cavities  but  a  pronounced  crack  run¬ 
ning  through  the  center  ;  often  this  crack  passes  along  through 
several  of  them,  and  it  is  always  rigorously  parallel  to  the 
direction  which  the  chain  or  line  of  spherulites  makes  (line  of 

*  Min.  Petr.  Mitt.,  xiv,  p.  569,  1895. 

f  Quar.  Jour.  Geol.  Soc.,  xliv,  p.  302,  1888. 


and  its  Spherulitic  Crystallization. 


103 


flow).  Where  one  has  a  cavity  the  ones  immediately  adjacent 
are  apt  to  have  the  crack.  In  some  places  this  crack  has 
opened  quite  widely  and  an  ellipsoidal  or  lenticular  cavity  is 
produced,  bordered  on  either  side  by  half  spherulites  extend¬ 
ing  into  the  glass.  The  bearing  of  these  facts  is  discussed  in  a 
later  place. 

Another  feature,  quite  similar  to  what  is  often  seen  in  acid 
volcanic  glasses,  is  the  presence  of  cracks  in  the  glass  immedi- 
diately  surrounding  the  spherulites.  These  commonly  start 
out  from  the  edge  of  the  spherulite,  and  especially  from  the 
place  where  two  tend  to  intersect,  and  then  curve,  sometimes 
splitting  and  forming  a  Y.  Usually  after  a  short  curve  they 
stop,  but  quite  often  they  continue  in  a  long  curve  concentric 
to  the  outer  surface  of  several  adjacent  spherulites  and  removed 
from  it  a  small  fraction  of  the  total  diameter  of  one  of  them 
(see  fig.  3).  It  is  owing  to  these  concentric  cracks  surrounding 
them  that  the  larger  spherulites  may  be  readily  broken  out  of 
the  glass  and  then  appear  with  a  thin,  smooth  skin,  like  var¬ 
nish,  coating  them. 

Fig.  3. 

v 


Fig.  3.  Spherulites  with  cavities  and  cracks. 

Stony  Material. — In  the  material  represented  in  the  collec¬ 
tion  there  are  some  pieces  of  a  white  lithoidal  or  porcelain-like 
appearance.  They  have  a  light  greenish  tone  of  color  and  a 
deeper  green  by  transmitted  light.  A  number  of  small  ves¬ 
icles  or  gas  pores  are  here  and  there  seen  in  the  material. 
Under  the  microscope  it  is  found  to  consist  of  glass  filled  with 
microlites  ;  some  of  rather  short,  minute  prisms  of  diopside 
with  inclined  extinctions  and  masses  of  feathery  and  fern-like 
forms  of  the  same  substance.  There  are  also  minute  fibers 
whose  extinction  appears  parallel  and  whose  exact  nature  is 
doubtful. 

Obsidian. 

One  of  the  most  interesting  features  of  the  collection  is  the 
presence  of  several  specimens  of  jet  black  artificial  obsidian, 
appearing  quite  similar  in  color,  luster,  and  fracture  to  the 
natural  obsidians  from  the  Yellowstone  Park,  Mono  Lake, 
Lipari  Islands,  and  other  well  known  localities.  Flo  wage  lines 


104 


L.  V.  Pirsson — Artificial  Lava-Flow 


or  directions  through  the  mass  are  indicated  by  parallel  chains 
of  little  points  around  which  the  glass  has  a  minute  but  differ¬ 
ent  fracture  and  which  thus  interrupt  the  broad,  smooth  sur¬ 
faces  of  the  usual  conchoidal  areas. 

The  appearance  of  this  black  glass  is  so  entirely  unlike  that 
of  the  ordinary  light-green  and  transparent  bottle  glass  that 
formed  the  flow  that  it  is  difficult  to  believe  it  is  merely  a  mod¬ 
ification  of  it,  though  positively  so  stated.  It  is,  however,  as 
will  be  presently  shown,  a  curious  modification  of  a  light-green 
transparent  bottle  glass,  though  whether  of  the  same  melt  as 
that  which  has  been  previously  described  as  containing  the 
splierulites  seems  doubtful.  The  doubt  rests  on  chemical  evi¬ 
dence.  The  first  glass  contains,  as  mentioned,  only  a  small 
amount  of  iron,  and  a  considerable  proportion  of  magnesia,  and 
diopside  has  crystallized  from  it;  the  qualitative  analysis  of 
the  obsidian  indicates  much  more  iron,  and  while  there  is 
abundance  of  lime,  magnesia  is  present  only  in  minutest  traces 
and  as  a  result  wollastonite  has  crystallized  out  in  a  particular 
layer,  as  will  be  described  later.  Unfortunately,  in  the  lapse 
of  time  since  the  material  was  formed  the  possibility  of  obtain¬ 
ing  more  exact  information  concerning  it  and  the  conditions 
under  which  it  was  produced  have  been  lost.  If  the  statement 
accompanying  the  collection  is  to  be  trusted,  then  magmatic 
differentiation  must  have  occurred,  which  seems  hard  to  believe. 

Considering  the  black  color,  it  seemed  at  first  as  if  the  glass 
must  have  been  of  some  other  melt  into  which  some  unusual 
ingredient  had  entered  which  colored  it  black,  or  if  a  part  of 
the  flow,  then  one  which  had  in  some  way  imbibed  a  coloring 
constituent.  Close  inspection  of  a  piece  of  this  obsidian  shows, 
however,  that  in  a  place  where  there  are  fractures,  or  cracks, 
penetrating  it,  if  the  mass  is  so  turned  to  the  light  that  the 
rays  entering  it  are  reflected  back  from  the  internal  surface  of 
the  crack  to  the  eye,  the  black  color  disappears  and  the  glass, 
between  the  crack  and  the  eye,  assumes  its  normal  sea-green 
transparent  aspect. 

The  black  color  is  due,  then,  not  to  a  chemically  diffused 
coloring  matter,  so  to  speak,  in  the  sense  that  iron  compounds 
color  glass  green,  manganese  purple,  or  cobalt  blue,  but  is  a 
mechanical  effect  of  some  kind,  owing  to  which  that  light 
which  strikes  it  and  is  not  immediately  reflected  from  the  outer 
surface,  penetrates  it  and  is  absorbed. 

Many  or  most  natural  obsidians  which  appear  black  are 
found  in  thin  section  to  be  composed  of  a  colorless  glass, 
swarming  with  specks,  or  trichites,  whose  exact  nature  is 
unknown,  but  which  many  believe  to  be  of  magnetite.  The 
idea  involved  is  this  :  An  obsidian  is  formed  because  the  effu¬ 
sion  and  cooling  have  been4  so  rapid  that  the  ordinary  rock  con- 


and  its  Spherulitic  Crystallization. 


105 


stituents  have  had  no  opportunity  to  begin  to  crystallize  before 
the  mass  stiffened.  But  experience  also  shows  that,  in  gen¬ 
eral,  the  more  rapid  the  cooling  the  greater  the  number  of 
centers  of  crystallization  which  will  be  set  up.  Now  if  any¬ 
thing  should  start  to  crystallize  in  magma  under  such  condi¬ 
tions  it  would  be  the  magnetite,  ordinarily  the  earliest  mineral 
of  importance  to  form,  and  it  would  be  therefore  distributed 
through  the  glass  in  the  shape  of  the  finest  dust  acting  mega- 
scopically  as  a  mechanical  pigment  and  coloring  it  black.  The 
red  color  that  many  obsidians  show  may  then  be  due  to  the 
complete  oxidation  of  the  magnetite  to  ferric  oxide. 

On  the  other  hand,  a  study  of  a  number  of  sections  of  vari¬ 
ous  obsidians,  and  also  of  the  literature,  seems  to  indicate  that 
the  color  may  not  be  wholly  due  to  magnetite.  Zirkel,*  in  dis¬ 
cussing  it,  considers  that  it  is  sometimes  inherent  in  the  glass 
(chemical  so  to  speak)  and  is  sometimes  due  to  inclusions  of 
minute  size,  which  he  describes.  In  the  latter  case  the  glass 
is  colorless,  and  in  the  study  of  several  obsidians  of  this  char¬ 
acter  the  writer  has  observed  that  the  microlites,  or  trichites, 
have  a  higher  index  of  refraction  than  the  surrounding  glass, 
as  shown  by  Becke’s  method.  On  lowering  the  objective 
beyond  the  focal  point  they  appear  black,  on  raising  it  above 
they  become  illuminated  and  appear  colorless.  Consequently 
they  are  transparent  and  not  magnetite.  The  greater  the 
number  of  these  incipient  crystallizations  there  are,  the  blacker 
and  less  transparent  the  glass  appears  megascopically.  The 
black  color  in  this  case  then  is  due  to  light  absorption.  In 
one  case  where  the  slender  microlites  were  arranged  in  parallel 
positions  in  streams,  it  was  noticed  that  the  section  possessed  a 
distinct  pleochroism ;  when  the  ray  vibrated  across  these 
streams  it  appeared  colorless  ;  when  the  long  diameters  were 
parallel  to  the  ray  it  was  distinctly  brown.  This  is  an  effect 
of  light  absorption  similar  to  that  previously  described  under 
spherulites  and  doubtless  produced  in  the  same  way.  The 
black  color  of  many  obsidians  then  appears  to  be  caused  by 
the  dispersion,  total  reflection,  and  absorption  of  light  due  to 
the  presence  of  innumerable  hosts  of  minute  crystalline  bodies 
of  a  higher  refractive  index  than  the  glass  in  which  they  lie, 
such  crystalline  bodies  being  themselves  colorless. 

In  the  case  of  the  artificial  obsidian  of  Ivane,  with  magnifi¬ 
cations  of  540  diameters,  the  microscope  reveals  the  presence 
of  bodies  of  indeterminate  nature.  They  are  so  minute 
that  no  definite  shape  can  be  assigned  to  them,  but  they 
give  a  vague  impression  of  being  rudely  octahedral.  On  lifting 
the  objective  they  become  strongly  illuminated,  on  lowering 
they  appear  black.  Thus  they  are  transparent  and  of  a  higher 

*Lelirb.  d.  Petrog.,  vol.  ii,  p.  271,  1894. 


106 


L.  V.  Pirsson — Artificial  Lava-Flow 


index  of  refraction  than  the  glass.  While  everywhere  freely 
sprinkled  through  the  field  of  view,  as  the  stars  appear  at 
night' in  the  sky,  they  cannot  be  said  to  swarm  in  the  sense 
that,  with  reference  to  their  own  diameters,  they  closely 
approach  one  another.  They  exert  no  perceptible  action  on 
polarized  light.  To  the  absorption  of  light  caused  by  the 
presence  of  these  minute  bodies,  as  with  some  natural  obsid¬ 
ians,  the  black  color  of  this  variety  of  the  Kane  glass  is 
ascribed. 

Partly  Crystallized  Obsidian. — In  one  place  there  is  a 
layer  of  the  obsidian  about  3  inches  thick,  which  has  partly 
crystallized.  The  black  glass  described  above  makes  a  rather 
clean  and  sharp  contact  with  it.  It  is  not  now  known  whether 
this  layer  was  above  or  below  the  pure  glass  one,  but  as  it  con¬ 
tains  impurities  on  the  side  opposite  the  glass,  and  also  on 
general  principles,  it  is  assumed  as  the  bottom  part  of  the 
obsidian.  In  the  specimen  it  has  a  stony,  not  glassy,  appear¬ 
ance  ;  is  of  a  very  dark  to  blackish  gray  color,  and  is  seen  to 
be  composed  of  a  sort  of  felt  of  innumerable  small  slender- 
bladed  crystals  2-4mm  long,  which  have  a  tendency  to  be 
arranged  parallel  to  the  extension  of  the  layer.  It  has  some¬ 
thing  of  a  rough  resemblance  to  some  hornblende  schists. 


Fig.  4. 


There  is  no  schistosity  in  the  fracture 
however,  which  is  rough  and  hackly. 
The  thin  section  under  the  microscope  is 
a  very  interesting  one.  It  is  composed 
of  a  pale  brown  glass,  filled  with  beau¬ 
tiful  crystallizations  of  the  mineral 
wollastonite.  While  in  general  the 
mineral  is  developed  as  usual  in  long  col¬ 
umnar  forms,  or  plates  parallel  to  the 
axis  of  symmetry,  its  growth  has  also 
been  in  such  aggregates  of  sheaf-like, 
rosette-like,  or  feathery  forms,  that  ac¬ 
cording  to  the  way  these  are  cut  differ¬ 
ent  effects  are  produced.  The  simplest 
case  is  shown  in  fig.  4,  which  gives  a 
section  parallel  to  b  (010),  across  one  of 
the  bladed  crystals.  The  faces  in  the 
zone  of  a  (100)  a  c  (001)  are  very 
sharply  developed,  as  may  be  seen  from  the  following  table 
of  angles  measured  against  the  cross-hairs  : 


Fig.  4. 
section  parallel  to  b  (010). 


Meas. 

Calc. 

a  (100) 

/'N 

c  (001) 

84° 

84° 

35' 

a'  (100) 

t  (101) 

50°  80' 

50° 

26' 

a  (100) 

/\ 

v  (101) 

42°  30' 

44° 

33' 

a  (100) 

/\ 

a  (102) 

70° 

69° 

55' 

and  its  Spherulitic  Crystallization.  107 

The  calculated  angles  are  those  given  by  Grosser*  and  the 
agreement  is  very  good,  except  in  v  (101),  which  was  not  so 
well  developed  as  the  other  faces.  The  three  cleavages  paral¬ 
lel  to  100,  001,  and  101  are  excellent,  as  represented  in  the 
figure,  their  excellence  being  in  the  order  given.  The  plane 
of  the  optic  axes  lies  in  the  clinopinacoid  and  the  bisectrix  a 
makes  an  angle  of  33°  in  the  acute  angle  / 3  with  the  vertical 
axis ;  Des  Cloiseaux  gives  32°  12'.  The  optical  character  is 
negative,  a  being  the  acute  bisectrix.  Since,  according  to 
Des  Cloiseaux,  the  optic  angle  in  air  2  E  =  about  70°,  it  follows 
that  one  optic  axis  emerges  almost  perpendicular  to  c,  001,  the 
other  at  an  angle  of  about  16°  to  a  (100).  Since  both  of  these 
are  good  cleavages,  it  follows  that  when  one  examines  the 
powder  made  by  crushing  the  material,  in  convergent  light 
under  the  microscope  with  crossed  nicols,  almost  invariably 
each  fragment  exhibits  the  locus  of  an  optic  axis  either  just  in, 
or  just  off,  the  field  of  view.  Since  the  a  (100)  cleavage  is 
the  best,  it  is  mostly  the  latter  case  that  obtains  and  in  the 
blades  the  long  direction  is  the  one  of  least  elasticity. 

While  these  crystallographic  and  optical  properties  prove 
the  nature  of  the  mineral,  its  identity  was  confirmed  by  the 
fact  that,  when  powdered,  it  dissolves  readily  in  hydrochloric 
acid  and  yields  gelatinous  silica. 

It  will  be  noted  in  the  drawing  of  the  crystal  shown  in  fig. 
4  that  the  a  (100)  faces  are  extended  in  a  thin  plate  forming 
re-entrant  angles ;  commonly  these  thin  plates  extend  far 
beyond  the  main  crystal  and  from  each  of  the  four  corners, 
tapering  off  indefinitely ;  the  cross  section  of  the  whole  then 
shows  an  H  with  the  vertical  legs  greatly  extended.  There 
may  be  other  cross  connections  producing  ladder-like  affairs. 
It  is  also  to  be  recalled  that  these  extend  as  sheets  perpendicu¬ 
lar  to  the  plane  of  the  drawing,  or  along  the  b  axis.  More¬ 
over  the  sheets  are  often  curved  and  numbers  of  these  extended 
plates  are  grouped  into  sheaves  or  rosette-like  groups,  and  thus 
a  variety  of  patterns  are  produced  as  these  are  cut  by  the  sec¬ 
tion  at  various  angles.  The  individual  filaments  as  they 
appear  in  the  section  are  commonly  curved  ;  if  examined  with 
a  very  high  power  it  is  seen  that  the  curve  really  consists  of  a 
series  of  short  minute  straight  pieces  of  crystal  with  wedge- 
shaped  cracks  between,  continuity  obtaining  only  along  one 
edge.  By  this  curving,  and  by  repeated  branching,  arborescent, 
or  plumose,  forms  are  produced,  and  in  places  the  glass 
between  crossed  nicols  appears  filled  with  them  and  seems  like 
masses  of  magnificent  ostrich  plumes  thickly  scattered,  the 
beauty  of  whose  effect  is  greatly  heightened  by  the  use  of  the 
sensitive  tint,  which  turns  them  brilliant  blue  and  yellow. 

*  Zeitschr.  f.  Kryst.,  xix,  p.  608. 


108 


L.  V.  Pirsson — Artificial  Lava-Flow 


These  plumose  forms,  which  are  of  remarkable  delicacy  and 
perfection,  resemble  the  growths  of  augite  in  basaltic  glass 
from  Hawaii,  described  by  E.  S.  Dana* ;  they  differ  from  the 
growths  in  the  pitchstone  of  Arran  in  being  much  larger,  more 
perfect,  and  in  the  curved  or  curled  character  of  the  stems, 
thus  resembling  plumes  rather  than  ferns.  They  never  form 
complete  spherulites,  though  in  places  the  thickly-grouped, 
feather-like  bunches  partly  resemble  them. 

Spherulitic  Crystallization . 

The  vast  majority  of  the  natural  spherulites  occur  in  acid, 
that  is  to  say,  siliceous  volcanic  glasses,  and  are  composed  of 
quartz,  or  feldspars,  or  of  these  two  minerals  in  various  pro¬ 
portions.  The  reason  for  this  is  because  it  is  especially  in 
magmas  of  this  nature  that  the  relation  between  viscosity  of 
magma  and  crystal  growth,  which  is  necessary  for  spherulitic 
crystallization  and  which  is  discussed  later,  is  apt  to  occur. 
Spherulite  crystallizations  may  occur  in  basic  magmas  and  are 
known  in  the  rocks  called  variolites,f  but  so  far  as  the  writer 
has  been  able  to  discover,  no  discussion  of  spherulites,  as  such, 
has  been  made  which  was  not  primarily  based  on  material  of 
the  kind  referred  to.  It  is  natural,  therefore,  that  in  these 
discussions  the  chemical  character  of  the  magma  involved  and 
the  nature  of  the  component  minerals  are  largely  held  respon¬ 
sible  as  determinant  factors  for  spherulitic  crystallization. 
Since  such  natural  magmas  contain,  as  is  well  known,  volatile 
constituents,  especially  water  vapor,  a  considerable  role  has 
been  ascribed  to  its  agency  in  this  connection.  Thus  Crossj: 
ascribed  the  origin  of  the  spherulites  in  a  rhyolite  studied  by 
him  to  the  presence  of  a  colloidal  condition  in  the  magma,  due 
to  the  antecedent  formation  of  masses  of  opaline  silica  contain¬ 
ing  the  elements  of  feldspar,  which  caused  their  formation 
and  globular  form.  Iddings§  also,  in  his  earlier  discussions  of 
spherulites,  suggests  that  water  vapor  plays  an  important  role 
in  rendering  certain  places  in  the  magma  less  viscous  and 
therefore  more  susceptible  to  molecular  movement  and  crystalli¬ 
zation.  This  would  explain  the  formation  of  crystalline  spher¬ 
ulites  in  some  places,  while  the  surrounding  magma  solidified 
as  glass  although  at  the  same  temperature.  He  says  :  “  Hence 
we  may  conclude  that  the  influence  of  the  absorbed  water- 
vapor  is  to  render  the  molecular  mobility  of  the  molten  magma 

*  This  Journal,  xxxvii,  441,  1889. 

f  Pirsson,  Petrog.  of  Igneous  Rocks  of  Little  Belt  Mts.,  20th  Ann.  Rep. 
U.  S.  Geol.  Surv. ,  Pt.  Ill,  p.  532,  1900. 

X  Constitution  and  Origin  of  Spherulites  in  Acid  Eruptive  Rocks,  Bull. 
Phil.  Soc.  Wash.,  xi,  411,  1896. 

§Bull.  Phil.  Soc.  Wash.,  vol.  xi,  p.  446,  etc.,  1891. 


and  its  Sjpherulitic  Crystallization. 


109 


greater  at  a  given  temperature  in  proportion  to  the  amount  of 
hydration,  thus  permitting  the  crystalline  arrangement  of  the 
molecules  in  places  of  greater  hydration,  while  the  surround¬ 
ing  less  hydrated  portions  are  becoming  too  viscous.” 

In  his  latest  discussion  of  spherulitic  crystallization,  Iddings* 
again  refers  the  conditions  controlling  it  in  acid  volcanic 
glasses  to  varying  amounts  of  water  vapor — as  u  probably 
dependent  on  viscosity,  as  affected  by  the  gas  contents  of  a 
magma.”  In  these  discussions,  however,  the  underlying  idea 
appears  to  be  not  so  much  an  explanation  of  the  assumption  of 
the  spherulitic  form,  or  habit  of  growth,  as  of  the  production 
of  local  conditions  which  would  favor  crystallization  and 
permit  the  formation  of  one  component  rather  than  another, 
for  in  spherulitic  growths  in  the  natural  volcanic  glasses  one 
must  deal  with  quartz  and  feldspar. 

Spherulitic  crystallization  in  the  ultimate  analysis  is  a  ques¬ 
tion  of  crvstal  form  or  rather  habit.  The  essential  thing  in  a 
typical  splierulite  is  that  from  a  common  center  crystals  grow 
in  all  directions  whose  elongation  is  excessive  as  compared 
with  their  breadth  and  thickness.  They  may  be  straight  rods, 
or  branching  rays,  or  blades,  or  assume  arborescent  shapes,  all 
of  which  occur  in  the  Kane  specimens,  but  always  tending  to 
elongate  forms,  thickly  crowded.  The  suggestion  of  Cross 
tends  to  assume  that  the  spherical  shape  or  rather  area  was 
defined  before  crystallization  actually  occurred  and  is  thus  an 
explanation  rather  of  the  outward  form  than  of  the  inward 
structure.  It  seems  highly  probable  that  the  degree  of  hydra¬ 
tion  in  the  natural  acid  glasses  affecting  the  viscosity,  as  sug¬ 
gested  by  the  writers  above,  plays  an  important  role  in  deter¬ 
mining  the  conditions  and  places  for  spherulitic  crystallization, 
but  in  the  Kane  glass  this  agency  was  not  present  and  the 
complication  of  having  two  mineral  substances  to  deal  with  is 
also  wanting.  The  question  here  is  simply  one  of  the  condi¬ 
tions  which  determined  the  assumption  of  a  certain  kind  of 
crystal  habit,  and  the  answer,  in  the  writer’s  opinion,  is  to  be 
found  in  the  degree  of  viscosity  which  had  been  attained  at 
the  time  when  the  saturation  of  the  solution  with  the  diopside 
molecule  reached  the  crystallizing  point.  Iddingsf  states  that 
the  habits  of  crystals  depend  in  a  large  degree  on  the  viscosity 
of  the  magma,  long  slender  prisms  and  branching  shapes  being 
commonly  developed  when  it  is  very  viscous,  though  he  does 
not  explain  why  this  is  so.  The  writer  offers  this  explanation 
for  the  slender  fibers  in  the  spherulites  of  diopside  in  the  Kane 
glass.  The  pyroxene  has  a  prismatic  cleavage  and  is  elongate 

*  Igneous  Rocks,  vol.  i,  pp.  231,  233,  1909. 

f  Igneous  Rocks,  vol.  i,  pp.  206,  216,  1909. 


110 


L.  V.  Pirsson— Artificial  Lava-Floic 

in  the  direction  of  this  cleavage.  It  is  not  singular  in  this 
respect,  for  most  minerals  tend  to 'be  elongate,  or  columnar,  in 
the  direction  of  pronounced  cleavages.  In  the  direction  normal 
to  the  cleavage  faces  the  molecular  network  has  a  lesser 
amount  of  cohesive  attraction  than  in  other  directions  and 
hence  the  cleavage.  We  may  therefore  imagine  that  during 
the  process  of  growth  the  amount  of  molecular  tension  or  pull 
exerted  by  the  crystal  upon  the  unorientated  molecules  in  the 
magma  is  greater  toward  the  end  face  than  toward  the  prism, 
or  cleavage,  faces.  Hence  the  supply  of  molecules  upon  it  is 
niore  rapid  and  the  crystal  grows  faster  in  this  direction.  If 
now  the  viscosity  of  the  magma  rises  to  such  a  degree  that  the 
tension  toward  the  side  faces  is  not  sufficient  to  overcome  i^ 
and  orientate  the  molecules  while  that  upon  the  end  face  is 
sufficiently  gleat,  then  the  crystal  will  extend  itself  like  a  long 
rod,  or  fiber,  until  the  growing  viscosity  puts  a  stop  to  further 
progress  in  this  direction  also.  The  crystallizing  effect  of  the 
fiber  end  would  also  be  aided  by  the  fact,  that  as  the  molecules 
fall  into  position  in  the  geometric  network,  or  change  from  the 
liquid  to  the  solid  state,  a  certain  amount  of  heat  is  liberated, 
tending  to  increase  the  mobility  of  the  still  unfixed  adjacent 
molecules  and  to  render  them  more  susceptible  to  crystallo¬ 
graphic  orientation.  The  crystals  then,  like  a  wire  with  a  hot 
end,  bore  out  into  the  stiffening  magma  in  all  directions,  and  as 
conditions  are  uniform  about  them  they  cease  simultaneously 
and  the  globular  shape  results.  This  relation  of  habit  between 
viscosity  and  cohesive  attraction  is  also  well  illustrated  by  the 
wollastonite  crystals  in  the  dark  glass.  They  extend  indefi¬ 
nitely  along  the  b  axis  perpendicular  to  which  there  is  no 
cleavage  and  are  also  tabular  to  the  a  (100)  face,  parallel  to 
which  the  best  cleavage  occurs.  In  the  fibers  of  feldspar  in 
splierulites  in  the  volcanic  rocks  the  elongation  in  the  major¬ 
ity  of  cases  is  parallel  to  the  clino  axis,  the  direction  of  the 
two  prominent  cleavages,  The  instances  cited  where  the  elon¬ 
gation  is  parallel  to  the  c  axis*  would  tend  to  show  that  the 
molecular  attraction  to  the  c  (001)  face’in  orthoclase  is  greater 
than  that  to  the  b  (010)  face.  The  fact  that  large  crystals  and 
especially  Carlsbad  twins  are  often  tabular  or  elongate  on  the  c 
axis  also  shows  this.  It  is  generally  considered  that  the  cleav¬ 
age  c(001)  of  orthoclase  is  better  than  that  of  b  (010),  and  this 
would  appear  to  make  these  cases  an  exception  to  the  general 
rule.  It  is  not  clear,  however,  judging  from  the  views  of  the 
writers  previously  stated,  that  viscosity  is  the  only  factor  to 
consider  in  these  cases,  as  it  is  in  the  Kane  glass,  as  a  delicate 
balance  between  several  different  factors  may  have  induced 

*  Iddings,  Spherulitic  Crystallization,  Bull.  Phil.  Soc.  Wash.,  vol.  xi,  p. 
456,  1891. 


and  its  Spherulitic  Crystallization . 


Ill 


this  particular  form.  Nor  does  the  writer  wish  to  affirm  as  a 
positive  rule  that  in  all  minerals  the  attraction  causing  growth 
is  less  towards  a  pronounced  face  of  cleavage  than  in  other 
directions.  It  does,  however,  appear  to  be  a  rather  general 

one. 

In  addition  to  the  elongation  of  the  fibers  in  spherulites 
there  is  also  the  branching  to  be  taken  into  account.  The 
more  rapid  growth  of  the  corners  and  edges  of  crystals,  due 
to  their  commanding  a  larger  portion  of  the  space  which  is 
supplying  the  crystallizing  material  and  thus  producing  branch¬ 
ing  and  skeleton  growths,  was  brought  to  attention  by  Leh¬ 
mann*  and  further  elaborated  .by  Bosenbuschf  and  needs  no 
further  discussion  here.  In  the  treatment  of  the  subject  by 
these  authors,  however,  it  is  tacitly  assumed  that  the  molec¬ 
ular  attraction  towards  all  faces  of  the  growing  crystal  is  the 
same,  or  at  least  there  is  not  mentioned  anything  to  the  con¬ 
trary,  and  the  view  previously  expressed  by  the  writer,  that 
this  may  be  different  on  different  faces,  brings  into  play  another 
factor. 

That  the  molten  glass  had  attained  a  considerable  degree  of 
viscosity  before  crystallization  began  is  clearly  indicated  bv 
the  spherulites  seen  in  figs.  B  and  C  of  the  plate,  since  it  was 
great  enough  to  support  these  denser  bodies  and  prevent  them 
from  sinking  to  the  bottom. 

That  the  formation  of  the  spherulites  was  a  comparatively 
rapid  process,  after  crystallization  once  started,  is  plainly  shown 
by  their  occurrence  in  the  manner  figured  in  A  of  the  plate. 
They  were  not  present,  of  course,  in  the  molten  glass  in  the 
furnace  before  breakage  occurred,  and  their  appearance  here 
in  fiow  lines  proves  they  had  formed  before  flowage  motion  of 
the  glass  had  ceased.  However  long  a  time  viscous  flowage 
may  continue  in  larger  masses  of  lava,  in  this  small  body  of 
glass  it  must  have  had  a  relatively  short  period. 

From  the  foregoing  discussion  it  appears  that  the  produc¬ 
tion  of  spherulites  in  an  anhydrous  molten  glass  depends  upon 
crystallization  starting  from  a  center  and  proceeding  out¬ 
wardly  in  all  directions  at  a  time  when  the  molten  solution  had 
attained  such  a  degree  of  viscosity  as  to  control  the  crystal 
habit  and  also  upon  the  composition  being  such  as  to  produce 
minerals  which  naturally  grow  in  columnar  forms.  The  time 
interval  between  this  point  and  that  where  the  fall  of  tem¬ 
perature  increases  the  viscosity  to  such  a  degree  as  to  prevent 
further  molecular  movement  evidently  controls  the  size  of 
the  spherulites.  In  the  case  of  the  huge  spherulites  described 
by  Cross;);  from  Colorado  this  time  interval  must  have  been 
relatively  long. 

*  IJeber  das  Wachstlium  der  Krystalle,  Zeitschr.  f.  Kryst.,  i,  462,  1877. 

f  Phys.  der  Min.,  1885,  p.  26,  4th  ed.,  vol.  1,  361,  1904.  %  Loc.  cit. 


112 


L.  V.  Pirsson — Artificial  Lava-Flow 


In  a  sheet  of  molten  glass  the  planes  of  cooling  descend 
vertically  into  the  mass  so  that  variations  of  temperature  from 
point  to  point  become  much  more  marked  in  this  direction 
than  in  a  horizontal  one.  It  is  also  true  that  in  the  change 
from  the  liquid  to  the  solid  condition  a  more  or  less  consider¬ 
able  contraction  of  volume  ensues.  Further,  the  force  of 
crystallization  is  very  powerful  within  the  distance  in  which 
it  acts,*  and  in  the  final  stage  of  viscosity  before  the  capa¬ 
bility  of  molecular  movement  ceases  the  tension  on  the 

c/ 

growing  crystal  faces  must  be  very  strong  and ,  per  contra ,  on 
the  adjacent  areas  of  unorientated  molecules.  Taking  these 
facts  into  consideration,  it  is  clear  that  especially  toward  the 
end  of  the  process  of  spherulitic  crystallization,  these  bodies 
and  the  glass  surrounding  them  will  be  subjected  to  stresses 
which  are  most  marked  in  vertical  directions.  Unable  to  with¬ 
stand  the  tension,  they  may  rupture,  giving  rise  to  horizontal, 
or  longitudinal,  cracks  or  even  be  pulled  apart  so  that  ovoid, 
or  spherical,  cavities  are  opened  within  them,  as  illustrated  in 
fig.  3.  Or  the  tension  between  them  and  the  surrounding 
glass  may  be  relieved  by  tangential,  or  radial,  cracks  in  the 
latter,  as  also  illustrated  and  described.  Thus  the  layers  of 
spherulites  spread  through  the  glass  in  bands  by  flowage  become 
elements  of  inherent  weakness  and  along  them  it  splits  into 
plates  and  thus  has  a  laminated  structure.  This  phenomenon 
is  also  noticed  in  natural  rhyolite  glasses,  like  those  from  Lipari 
and  the  Yellowstone  Park,  which  cleave  readily  along  the 
bands  of  spherulites.  Iddings,f  in  discussing  the  laminated 
nature  of  such  rocks,  attributes  it  to  non-homogeneity  in  dif¬ 
ferent  parts  of  the  magma,  produced  by  variable  amounts  of 
contained  water  vapors,  which  unlike  portions  by  the  spread¬ 
ing  out  action  of  flowing  lavas  become  distributed  in  thin 
sheets.  Hence  arise  layers  of  different  degrees  of  consistency, 
crystalline  character,  etc.,  which  condition  the  banded  struc¬ 
ture  and  cause  lamination.  The  Kane  glass,  however,  shows 
that  the  same  lamination,  though  not  perhaps  in  so  high  a 
degree,  can  be  formed  in  the  cooling  of  an  anhydrous  magma 
and  that  the  water  vapor,  except  where  it  causes  layers  of  bub¬ 
bles,  as  in  pumice,  must  be  an  indirect  rather  than  a  direct 
agent  in  producing  lamination  in  that  it  promotes  more  favor¬ 
able  conditions  for  crystallization. 

Differentiation . — In  the  clear  glass  containing  relatively 
few  small  spherulites  there  is  a  non-liomogeneous  quality  which 
shows  itself  by  drawn  out  streaked  lines  and  layers,  these 
being  nowise  different  in  color  or  consistency,  but  having  dif- 

*  Becker  and  Day,  Linear  Force  of  Growing  Crystals,  Wash.  Acad.  Sci. 
Proc.,  v o] .  vii,  p.  283,  1905. 

f  This  Journal,  vol.  xxxiii,  p.  43,  1887  ;  Igneous  Rocks,  vol.  i,  p.  243,  1909. 


and  its  Sjpherulitic  Crystallization . 


113 


ferent  refractive  indices,  and  thus  becoming  visible  by  tlieir 
action  on  light,  in  the  same  manner  that  heated  air-currents 
are  visible  above  a  source  of  heat,  or  differing  currents  of 
salt  laden  solutions  are  seen  in  a  liquid.  These  streaks  are 
small  and  line  and  are  parallel  to  one  another,  and  to  the 
chains  of  spherulites,  in  the  direction  of  How.  Since  the 
refractive  index  varies  in  the  different  layers  there  must  be  a 
difference  in  composition  to  occasion  it.  The  difficulty  of 
obtaining  glass  of  uniform  composition,  through  a  tendency  to 
separate  into  unlike  portions,  is  one  well  known  to  makers  of 
lenses  and  other  users  of  optical  glass  and  has  been  frequently 
described.  It  needs  no  further  mention  here  beyond  the  com¬ 
ment  that  this  case  adds  another  to  those  which  have  been 
cited  by  others  as  a  proof  that  the  instability  of  molten  silicate 
solutions  furnishes  a  presumptive  proof  of  the  possibility  of 
magmatic  differentiation  on  a  larger  scale. 

If  we  accept  the  statement,  which  cannot  unfortunately  be 
now  verified,  that  the  black  obsidian  is  really  a  part  of  the 
same  glass  flow,  a  more  striking  instance  of  differentiation  is 
shown  in  that  the  clear  glass  is  rich  in  magnesia  and  poor  in 
iron  oxide  while  in  the  obsidian  the  reverse  is  the  case.  This 
difference  explains  very  clearly  why  diopside  crystallized  out 
in  the  one  and  wollastonite  in  the  other.  But  such  a  move¬ 
ment,  provided  we  suppose  the  original  molten  solution  to 
have  been  homogeneous,  whereby  magnesia  and  ferrous  oxide 
are  concentrated  in  opposite  directions,  while  one  would  hesi¬ 
tate  to  say  it  was  impossible,  from  all  our  experience  gained 
in  the  study  of  rock-masses  in  which  these  oxides  move  together, 
seems  very  unlikely  to  say  the  least.  And  on  the  other  hand, 
we  do  not  know,  in  the  production  of  glass  on  so  large  a  scale, 
whether  a  sufficiently  high  temperature  was  maintained  long 
enough  to  permit  the  molten  solution  to  assume  conditions  of 
uniformity  throughout  its  mass,  if,  on  account  of  a  difference 
of  materials  used  in  the  making,  uniformity  was  not  present  to 
begin  with.  Viewed  from  this  standpoint,  it  is  possible  that 
the  obsidian  was  part  of  the  same  flow.  At  all  events,  the 
material  seems  to  indicate  the  possibility  of  obtaining  in  this 
direction,  artificially,  some  light  on  the,  at  present,  mysterious 
process,  or  set  of  processes,  known  as  differentiation. 

Composition  of  Minerals. — The  use  of  the  terms  diopside 
and  wollastonite  in  this  paper  is  of  course  only  a  general  one 
and  it  is  not  assumed  that  these  substances  are  necessarily 
present  in  a  pure  condition.  From  the  splendid  results  which 
have  been  achieved  in  recent  years  by  Dr.  Day  and  his 
co-workers  in  the  Geophysical  Laboratory  of  the  Carnegie 
Institution  concerning  the  properties  of  the  lime  and  magnesia 
silicates,  and  the  conditions  under  which  they  are  formed,  and 

Am.  Jour.  Sci. — Fourth  Series,  Vol.  XXX,  No.  176. —  August,  1910. 

8 


114 


L.  V.  Pirsson — Artificial  Lava-Flow. 


which  have  been  published  in  this  Journal,*  it  has  been  shown 
so  clearly  that  diopside  and  wollastonite,  as  produced  from 
molten  solution,  may  contain  variable  amounts  of  lime  and 
magnesia  metasilicates  in  solid  solution,  that  it  would  seem 
quite  possible  that  the  diopside  in  the  Kane  glass  is  far  from 
being  pure  in  composition.  The  analytical  test  proves  that 
the  wollastonite  is  relatively  a  pure  compound,  and  the  optical 
data,  so  far  as  they  go,  agree  with  this  view. 

It  is  evident  from  the  works  quoted  that  the  temperature  of 
the  glass,  when  the  wollastonite  crystallized,  must  have  been 
lower  than  1190°,  since  this  is  the  inversion  point  of  the 
mineral  to  pseudo-wollastonite,  which  alone  exists  above  that 
temperature  and  up  to  1512°,  its  melting  point.  The  melt¬ 
ing  point  of  diopside  is  1380°  and  the  temperature  of  the 
molten  solution  was  therefore  originally  above  this  point;  this 
of  course  is  to  be  expected,  even  though  a  flux  is  used  to  help 
carry  the  quartz  sand  and  lime  carbonate  into  solution. 

Summary. — -The  study  of  the  accidental  flow  of  molten 
bottle-glass  at  Kane,  Penn.,  has,  brought  out  the  following 
chief  points : 

That  spheruiites  of  varying  size  and  character  and  consist¬ 
ing  of  diopside  may  be  formed  in  an  anhydrous  molten  solu¬ 
tion  by  rapid  cooling. 

That  the  spherulitic  type  of  crystallization  appears  to  be 
conditioned  by  the  relation  of  crystal  habit  and  properties  to 
the  viscosity  of  the  magma.  The  spheruiites  are  of  rapid 
growth. 

That  the  brown  color,  which  many  spheruiites  exhibit  by 
transmitted  light,  is  a  phenomenon  of  light  absorption. 

That  obsidian  may  be  artificially  produced  from  a  clear  glass 
and  that  its  black  color  is  a  phenomenon  of  light  absorption. 

That  artificial  wollastonite  may  exhibit  certain  characters 
which  are  described. 

o 

Sheffield  Scientific  School  of  Yale  University, 

New  Haven,  Conn.,  April,  1910. 

*Day,  Shepherd  and  Wright,  The  Lime-Silica  Series  of  Minerals,  vol.  xxii, 
pp.  265-302,  1906.  Allen,  White  and  Wright,  On  Wollastonite  and  Pseudo- 
wollastonite,  Polymorphic  Forms  of  Calcium  Metasilicate,  vol.  xxi,  pp.  89- 
108,  1906.  Allen,  White,  Wright  and  Larsen,  Diopside  and  its  Relation 
to  Calcium  and  Magnesium  Metasilicates,  vol.  xxvii,  pp.  1-47,  1909. 


Am.  Jour.  Sci . ,  Vol.  XXX,  1910 


Plate  I 


Spherulites  in  Artificial  Glass 


